XP Tablet PC Edition should have the on-screen keyboard available for the login prompt. I think there is a button that should bring it up or something, but I haven't had a tablet to play with in a long time :/

If you set a computer for automatic logon, anyone who can physically gain access to the computer can also gain access to everything that is on the computer, including any network or networks that the computer is connected to. Additionally, if you turn on automatic logon, the password is stored in the registry in plain text. The specific registry key that stores this value is remotely readable by the Authenticated Users group. Therefore, only use this setting if the computer is physically secured and if you make sure that users who you do not trust cannot remotely see the registry.

You can use Registry Editor to add your log on information. To do this, follow these steps: